A cement bond log (CBL) plays an important role when evaluating the quality of cement behind casing and determining well integrity, which in turn helps to ensure zonal isolation and wellbore protection. Logging-while-drilling (LWD) sonic tools provide an economical solution for locating the top of cement and determining the cement bond index, particularly for highly deviated wells due to the ease of conveyance. In general, LWD tools can make time-lapse measurements and save rig time and cost by eliminating separate wireline runs. However, LWD sonic measurements are often contaminated by the tool-borne wave mode and road noise, resulting in biased casing wave amplitude and attenuation estimates. Using these biased measurements directly can yield an unreliable bond index (BI), particularly for zones where the tool-borne waves are significantly stronger than casing arrivals. These challenges can be overcome as demonstrated in a field data example with both free casing and well-bonded zones.
The paper describes a quantitative cement evaluation method developed using LWD monopole data. The method uses the characteristics of LWD sonic tools—the tool waves have an intrinsic stop band in excitation, and the tool mode speed is slower than the casing wave.
During the first step of the process, band-pass filters are developed and applied to extract casing waves in the intrinsic stop band of the tool mode. The waveforms of a single-shot data recorded by different receivers are then stacked to a reference receiver with different weights to further suppress the tool-borne wave arrivals and increase the signal-to-noise ratio (SNR) of the desired signals. The weights are calculated according to the speed differences between the casing and tool waves. The remaining tool waves after the array processing are predicted by multi-shot data, including both a free-casing zone and a well-bonded zone. A simplified tool-casing wave model is developed and applied to describe and predict the interference phenomenon between the casing arrival and the tool arrival. The tool wave signals are calculated from the well-bonded data by subtracting the casing wave predictions calculated from the free-casing data with modeling predictions. The true casing wave amplitude is estimated from the filtered waveforms and calibrated by subtracting the tool wave predictions. A BI log is then calculated from the calibrated casing wave amplitude.
This new processing technique was applied to a field data example exhibiting both free-casing and well-bonded zones. Both the CBL amplitude in dB and BI log are extracted from the LWD sonic data and compared with BI estimated from a wireline segmented bond tool (SBT). A good match is obtained over the entire log despite the fact that a casing pressure test was conducted between the acquisition of the LWD sonic and wireline data, which might have created micro-annuli detected by the SBT measurements.
The proposed approach promises to be a significant step toward deriving a quantitative CBL from LWD sonic monopole data.